The microwave down-conversion is one of the key technologies for microwave signal processing. The function is to convert the frequency of the RF signal received by the antenna to the intermedia frequency range that can be processed subsequently. Microwave down-conversion technology has extensive and important applications in satellite communications, wireless communications, radar and so on. The traditional microwave down-conversion technology based on electrical method is mainly realized by the nonlinear functions of electronic devices such as diodes or field effect transistors. The processing range and bandwidth might be limited by the performance of the electrical device. As the microwave frequency increases, the conversion efficiency based on the electrical method might be reduced, and the phase noise may be deteriorated, which greatly impacts the quality of the down-converted signal.
The microwave photonic technology by combining the complementary advantages of the photonic and microwave technology provides a promising solution for microwave down-conversion processing. The microwave photonic down-conversion technology can fully exert the advantages of photonic technology, such as large bandwidth, low loss, tunablility and multiplexing. It has good immunity to electromagnetic interference and has great potential in reducing the size and power consumption of down-conversion system, and enhancing the frequency conversion performance.
The microwave photonic frequency down-conversion scheme based on cascaded LiNO3 modulators has been studied in prior art [1] (M. M. Howerton, R. P. Moeller, G. K. Gopalakrishnan, and W. K. Burns. “Low-biased fiber-optic link for microwave downconversion”, IEEE Photonics Technology Letters, Vol. 8, No. 12, pp. 1692-1694, December 1996). The external electrical local oscillator (LO) signal and the RF signal are modulated onto the optical carrier from the laser through two cascaded LiNO3 modulators, respectively. Then, the intermediate frequency (IF) signal is obtained by frequency beating between the electrical LO signal and the RF signal carried on the optical carrier in the photodetector. The external electrical LO source and two LiNO3 modulators are needed, which may increase system complexity. On the other hand, the gain characteristics of the frequency down-conversion would be degraded due to the added optical insertion loss of the fiber connection between two modulators.
The microwave photonic frequency down-conversion technology based on a single integrated double parallel Mach-Zehnder modulator (DP-MZM) has been investigated in prior art [2] (Erwin H. W. Chan and Robert A. Minasian. “Microwave photonic downconverter with high conversion efficiency”, IEEE Journal of Lightwave Technology, Vol. 30, No. 23, pp. 3580-3585, December 2012). The DP-MZM consists of two sub MZM modulators (MZM1 and MZM2) embedded in the main Mach-Zehnder interference arms. The external electrical LO signal and the RF signal are respectively modulated to the lightwave output from the laser through MZM1 and MZM2. Compared with the prior art [1], the scheme implements two independent intensity modulation in one integrated device. The integrated optical waveguide shortens the transmission interval between discrete devices, the number of fiber couplings are reduced, and the optical loss could be reduced. The system structure is simplified, which is beneficial to improve the down-conversion gain. However, an external LO source is still needed, and the external DC bias controllers for MZM1 and MZM2 are needed to stabilize the carrier suppression operation condition, which may increase the system complexity.
The scheme based on the optoelectronic oscillator (OEO) composed of a single integrated double driver Mach-Zehnder modulator (DD-MZM) for microwave photonic frequency down-conversion has been studied in prior art [3] (Zhenzhou Tang, Fangzheng Zhang, and Shilong Pan. “Photonic microwave downconverter based on an OEO using a single dual-drive Mach-Zehnder modulator”, Optics Express, Vol. 22, No. 1, pp. 305-310, January 2014). In the scheme, the LO signal is generated by the OEO consisting of a RF input port of the DD-MZM, optical fiber, photodetector and electrical filter. The RF signal received by the antenna is modulated on the lightwave from the laser through another RF input port of the DD-MZM. The IF signal is obtained by the frequency beating between the RF signal and the LO signal generated by the OEO via the second photodetector. No external electrical LO signal source is needed in the scheme. The modulation function of the LO signal generated by the OEO loop and the RF signal received by the antenna is implemented by a single integrated DD-MZM, which simplifies the system structure. However, the LO signal frequency is selected by the electrical filter, which may be difficult to be tuned flexibly. On the other hand, the RF signal can also be recovered after photoelectric conversion, which is unable to achieve the complete RF isolation.